The present invention is directed generally to a method and apparatus for forming refractive index gratings in a medium, and more particularly to a method and apparatus that uses polarization control of the exposing beams for forming refractive index gratings in the medium.
Certain optical media, including at least some silica-based waveguides, can be modified by exposure to electromagnetic radiation in an appropriate spectral range. The exposure of the optical media may induce refractive index changes affecting the optical properties in the illuminated portions of the optical medium.
Refractive index changes can be induced in photosensitive optical media. The photosensitivity means that the incident electromagnetic radiation interacts, at least to some degree, with the matter constituting the medium, implying an absorption of the electromagnetic radiation in the medium. Hence, the photosensitivity of the optical medium, and the strength of the changes in the refractive index, are dependent of the chemical composition of the medium. Germanosilicates are widely used for photosensitive waveguides, but other materials and/or other dopants than germanium may also give the desired photosensitivity.
Ideally, the photo-induced change in the refractive index, xcex94n, is linearly dependent upon the fluence of radiation on the photosensitive medium. The fluence, xcfx86(r), is the amount of energy per unit area and is defined as xcfx86(r)=∫I(t) dt, where I(t) is the intensity of the applied radiation at time t, for a position r. Hence both the fluence and the intensity are used for characterizing the radiation. The dependency of xcex94n on xcfx86(r) diverges from the ideal linear dependency for some material compositions and/or for high intensities.
If the incident radiation field forms a pattern on the medium, the induced changes in the refractive index may form a corresponding pattern. For example, an interference pattern in the incident radiation field may form a periodic pattern in the photosensitive medium, such as a periodic pattern forming one or more Bragg gratings. FIG. 1A illustrates a typical method for writing a periodic index pattern in a waveguide such as an optical fiber or a planar waveguide. A laser beam 102 of actinic radiation is directed through a phase mask 104, through a cladding layer 108 of the medium 106 and into the core 110. The phase mask 104 generates an interference pattern with a period half that of its surface relief pattern 105. Index of refraction changes in the core 110 occur predominantly at the bright fringes of the interference pattern, thus creating a periodic variation 112 in the refractive index grating, also referred to as a grating, in the core 110. The laser beam 102 may be translated along the medium 106 in order to write a longer grating 112, for a given width of beam 102. The actinic radiation used is typically UV or near UV radiation, but other wavelength ranges may be used, depending on the wavelength sensitivity of the photosensitive species in the core 110.
The refractive index grating 112 may operate as a spectrally selective reflector or transmitter for electromagnetic radiation propagating along the core 110. In general, the spectral response of the refractive index grating 110 is determined by a number of different parameters, including the shape of the grating, the period of the refractive index modulation, the variation in the period (also referred to as chirp), phase relations, amplitude modulations and the like.
In a simple approach, the shape and size of the grating 112, may be described as the effective refractive index, n(r), as a function of position. In a simple form, the effective refractive index may be given as n(r)=n0+xcex94n(r), where n0 is independent of position. This expression may not, however, be adequate in all contexts since the refractive index of a medium may also depend on the frequency and the polarization of the light propagating in the medium, as well as number of other parameters.
Various methods for controlling the writing of the refractive index grating have been proposed and utilized in the prior art. These methods have been based on such parameters as control of the laser intensity, scan speed, pulse rate, or using a controlled vibration of the phase mask or sample, or some combination of these.
A frequently used technique to improve the spectral response when writing refractive index gratings in waveguide structures is apodisation. A frequently encountered problem during apodisation is chirp. The period xcex9 of a periodic grating is the optical distance between amplitude peaks in the periodic structure. However, the optical distance between two points is also dependent on the mean refractive index in the region between the two points. Hence, when the refractive index modulation is written, the mean index and thereby the optical distance and the period may change throughout the grating structure. Since the amplitude of the grating modulation typically varies over the periodic structure, for example to obtain apodization, the mean index and hence the period seen by radiation propagating in the medium varies, and the grating is subject to xe2x80x9cchirpxe2x80x9d. This is illustrated in FIG. 1B wherein the oscillating curve 152 shows the periodic structure and the solid curve 154 represents the mean index change along the grating.
U.S. Pat. No. 5,830,622 discloses a method for modulating the mean index by providing a method for forming an optical grating using two steps. In the first step, the periodic grating structure is written in a glass. Subsequent or prior thereto, a region concomitant to the grating structure is illuminated with radiation having a predetermined spatial distribution, intensity, wavelength etc. in order to raise and/or modulate the mean refractive index of the region. A disadvantage of the method for controlling the writing of refractive index gratings disclosed in U.S. Pat. No. 5,830,622 is that two separate exposures are required.
WO 97/21120 discloses a method for writing a refractive index grating by creating an interference pattern in the medium between two beams. The two beams are formed from one beam by deflecting parts of the beam to generate two beams, which are controlled simultaneously by overlapping the beam paths of the two beams.
It is a disadvantage of the existing methods for controlling the writing of refractive index gratings that the material is non-reciprocal meaning that the photosensitivity is changed during the first exposure and the change in photosensitivity is a non-linear function of the locally applied fluence. Therefore it is nearly impossible to raise the mean refractive index to a constant level and/or maintain the desired refractive index amplitude simultaneously.
It is a disadvantage of the other existing vibration-based methods for controlling the writing of refractive index gratings that interferometric (submicron) stability is needed for the entire writing set-up.
The present invention is directed to a system and method for controlling the writing of refractive index structures such as gratings in an optical waveguide, and in particular to a system and method that may use only a single writing step. The present invention is further directed to a system and method for controlling the writing of refractive index structures that do not require interferometric stability of the control elements.
One embodiment of the invention is directed to a method for changing a refractive index of a first part of a medium. The method includes simultaneously illuminating the first part of the medium with at least part of a first beam of electromagnetic radiation and at least part of a second beam of electromagnetic radiation, wherein the first beam has a first polarization state and a first wavevector and the second beam has a second polarization state different from the first polarization state, and a second wavevector different from the first wavevector.
Another embodiment of the invention is directed to a method for changing a refractive index of a first part of a medium, that includes providing first and second beams of electromagnetic radiation, the first beam having a first polarization state and a first wavevector, the second beam having a second polarization state different from the first polarization state, and a second wavevector different from the first wavevector. The method also includes illuminating a diffractive optical element by at least a part of the first beam and a part of the second beam so as to diffract parts of the first and second beams, and positioning the medium in relation to the diffractive element so as to illuminate the first part of the medium by the diffracted parts of the first and second beams.
Another embodiment of the invention is directed to a method for inducing a refractive index grating in a medium that includes generating a substantially polarized light beam, dividing the first beam into a second beam and a third beam using a polarizing beamsplitter, the second and third beams being mutually orthogonally polarized and having respective second and third wavevectors, the second wavevector being different form the third wavevector. The method also includes substantially extinguishing the third beam, generating a second diffraction pattern by illuminating a diffractive optical element with the second beam and illuminating a first part of the medium with the first diffraction pattern so as to induce a first refractive index grating in the medium, the first refractive index grating having a first period xcex81. The method also includes substantially extinguishing the second beam, generating a third diffraction pattern by illuminating a diffractive optical element with the third beam, and illuminating a second part of the medium with the third diffraction pattern so as to induce a second refractive index grating in the medium, the second refractive index grating having a second period xcex92. The method also includes controlling a phase between the first refractive index grating and the second refractive index grating by controlling a distance between the diffractive optical element and the medium.
Another embodiment of the invention is directed to a method of changing the refractive index of a medium that includes illuminating a first part of the medium with a first set of diffracted light beams produced by a diffractive optical element so as to induce a first refractive index grating in the medium, and illuminating a second part of the medium with a second set of diffracted light beams produced by the diffractive optical element so as to induce a second refractive index grating in a second part of the medium. The method also includes controlling a phase difference between the first and second refractive index gratings by adjusting a working distance between the medium and the diffracting optical element.
Another embodiment of the invention is directed to a system for changing a refractive index of at least part of a medium. The system includes means for generating first and second beams of electromagnetic radiation having first and second wavevectors respectively, the first beam being polarized substantially orthogonally to the second beam, and diffracting means for generating a first set of diffracted beams with a first polarization state when illuminated by the first beam and for generating a second set of diffracted beams with a second polarization state when illuminated by the second beam. At least the part of the medium is positioned so as to be illuminated by at least part of one of the first and second sets of diffracted beams.
Another embodiment of the invention is directed to a system for changing refractive index of at least part of a medium that includes a light generating unit producing a first polarized light beam having a first wavevector and a second polarized light beam having a second wavevector different form the first wavevector, the first beam being polarized substantially orthogonally to the second beam. The system also includes a diffractive optical element disposed in the first and second beams to generate a first set of diffracted beams from the first beam and a second set of diffracted beams from the second beam, the at least the part of the medium being positioned so as to be illuminated by at least part of one of the first and second sets of diffracted beams.
The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and the detailed description which follow more particularly exemplify these embodiments.